The present invention relates to a digital broadcast receiver, suitable for use in digital satellite broadcasting, and a method for controlling reception thereby, and more particularly to a technique making possible stable reception even where the carrier to noise (C/N) ratio of received signal is low.
Digital satellite broadcasting is coming into expanding use. Compared with analog broadcasting, digital satellite broadcasting, which uses the Moving Picture Experts Group Phase 2 (MPEG 2) system or the like for data compression, is more resistant to noise and fading, and capable of transmitting signals of higher quality. It can also facilitate multichannel communication and cost reduction per channel. Furthermore, it is capable of providing not only video and sound signals but also such other services as data transmission.
Generally, in receiving a television broadcast, when the C/N ratio drops, received images are disturbed, and picture quality deteriorates. As shown in FIG. 4, while picture quality gradually deteriorates in analog broadcasting as the C/N ratio drops, it is sharply degraded in digital broadcasting when the C/N ratio drops to a certain level.
The reason why picture quality is degraded by a drop in C/N ratio in such a manner in digital broadcasting is that error correction is processed in digital satellite broadcasting. That is, in digital satellite broadcasting, as shown in FIG. 5, when the C/N ratio drops, the bit error rate (BER) rises. Unless the BER rises to a certain level, error correction is possible on the part of the receiver.
In multichannel digital satellite broadcasting referred to above, section data are multiplexed and transmitted together with video data and audio data, which are program data. Section data include data necessary for conditional accessing and the electronic program guide (EPG) as well as data called program specific information (PSI), which are required for channel selection. The transmission rate for each type of data is, for example, 6 Mbps for video data, 256 Kbps for audio data or 10 Kbps for section data. In a digital satellite broadcast receiver, when receiving a multichannel digital satellite broadcast, a demultiplexer first detects section data and the detected section data are temporarily stored in a buffer memory connected to the demultiplexer. In the section data stored in such manner, the PSI data necessary for channel selection are read by a microcomputer. The PSI data include table data such as a network information table (NIT), a program association table (PAT) and a program map table (PMT). Particulars of transmission (such as the plane of polarization, carrier frequency and convolution rate) for each carrier are stated in the NIT. The PAT lists information on channels in the carrier and the packet ID (PID) of the PMT that represents the contents of each channel. The PID of the transport packet of video and audio data that constitute each channel is stated in the PMT. Accordingly, by referring to the PSI, it is possible to identify the carrier used for transmission on the channel selected by the user and the PID the transport packet of the video and audio data on the channel has. By referring to the PSI, the microcomputer identifies the carrier that is to be received, and controls the tuner accordingly. Further, by referring to the PSI, the microcomputer also identifies the PID of needed video and audio data, and controls the demultiplexer to extract the transport packet having this PID. The transport packet thereby extracted is decoded by an MPEG decoder, and video signals and audio signals are obtained.
In the multichannel digital satellite broadcast receiver described above, since the probability of error occurrence is one per 10,000 bits when BER is 10xe2x88x924 for example, 600 errors per second in video data, 25 to 26 errors per second in audio data and one error per second in section data are likely to occur. Accordingly, even if the reception of video and audio data is impossible (error correction cannot be performed), the reception of section data may be possible to some extent (the error correction can be performed).
In a conventional digital satellite broadcast receiver, the operation of a receiver is controlled by using a one bit lock/unlock signal, which is the output of a quadrature phase shift keying (QPSK) demodulator. This lock/unlock signal is a signal that indicates the possibility or impossibility of QPSK demodulation; when the QPSK demodulation is possible, a locked mode is selected, and when the QPSK demodulation is impossible, an unlocked mode is selected.
FIGS. 6A to 6F show the operation of a conventional digital satellite broadcast receiver at a low C/N ratio. As shown in FIGS. 6A to 6C, when the C/N ratio drops and the BER rises to 10xe2x88x9210 for example, error correction cannot be performed by a forward error correction (FEC) decoder, and pictures consisting of video data cannot be displayed. When the C/N ratio further drops and the BER rises to 10xe2x88x922 for example, reception becomes impossible because the QPSK demodulator becomes unable to perform demodulation. Then, the lock/unlock signal changes from lock to unlock. Thus, when the C/N ratio is going down, transition to the unlocked mode takes place some time after it becomes impossible to display pictures (after the C/N ratio has dropped by 2 decibels for example). When the lock/unlock signal changes to unlock, the receiver clears the section data that have been stored in the buffer memory, connected to the demultiplexer, and have been used.
As shown in FIGS. 6D to 6F, when the C/N ratio rises in the unlocked state and the BER drops to 10xe2x88x922 for example, the QPSK demodulator resumes the ability to perform demodulation, and the lock/unlock signal shifts from unlock to lock. As the section data are receivable then, they are acquired. Moreover, when the C/N ratio is further enhanced and the BER drops to 10xe2x88x9210 for example, error correction can be performed by the FEC decoder, and it becomes possible to display pictures consisting of video data. Accordingly, a channel is selected by using the section data, which have been detected when the mode shifted from unlock to lock, and video and audio data are outputted. Thus, when the C/N ratio is going up, the section data become receivable, and after a while (after the C/N ratio has risen to 2 decibels for example), it becomes possible to display pictures.
In the conventional digital satellite broadcast receiver described above, as the C/N ratio is liable to fluctuate particularly in the rain, alternate mode shifts from unlock to lock and vice versa may happen. This fluctuating state would disturb and destabilize the operation of the digital satellite broadcast receiver. Furthermore, section data are acquired in an unstable state of the C/N ratio. Accordingly, correct section data might not be obtained, possibly inviting reception disturbance.
The present invention is attempted in view of these problems, and it is an object of the present invention to provide a digital broadcast receiver and a method for controlling reception thereby, both capable of ensuring stable operation even when the C/N ratio is low.
According to the present invention, since the configuration is to give the receiving operation of a digital broadcast receiver a hysteretic characteristic against changes in the C/N ratio, stable reception can be achieved even if the C/N ratio is at a low level.
A digital broadcast receiver according to the invention receives digital broadcast signals in which section data are multiplexed with multichannel program data, and is provided with a controlling means for effecting control to give its receiving operation a hysteretic characteristic against changes in the C/N ratio.
By a method for controlling reception by a digital broadcast receiver according to the invention, reception having a hysteretic characteristic against changes in the C/N ratio is performed when the digital broadcast receiver receives digital broadcast signals in which section data are multiplexed with multichannel program data.
According to the present invention, a digital broadcast receiver, from a state in which reception is impossible, starts reception after the C/N ratio rises to a level where reception is possible and further reaches or surpasses a prescribed level.